Mine has to be Dragon Quest: Rocket Slime, a DS spin off of the Dragon Quest series that sees you playing as a slime operating a tank and rescuing the people from your town. You run around the overworld, collecting items to use as ammunition and saving money to upgrade your tank. The art and music are just as great as you’d expect from the Dragon Quest series. It made fantastic use of the DS’s dual screens. It’s also written for a younger audience, so a lot of it is just really silly and fun! Try it out for sure, I’m so sad there’s no sequel :(
I’ve just recently downloaded an emulator and started replaying it. Going quite slowly through it, but I hope eventually to actually finish it.
As a kid I got up to the Dark Genie twice (on two separate playthroughs), but I never actually managed to beat him.